1. Field of the Invention
The present invention pertains to a vapor phase epitaxial growth technology of a single crystalline gallium nitride (GaN) based compound semiconductor, more particularly to a method of fabricating a single crystalline substrate (wafer) of GaN based compound semiconductor (hereinafter referred as xe2x80x9cthe GaN based semiconductorxe2x80x9d) having an excellent crystallographic quality and to a method of manufacturing a semiconductor device using this single crystalline wafer of GaN based semiconductor.
2. Description of the Related Art
Recently, blue light emitting semiconductor devices and electronic devices using a single crystalline GaN based semiconductor as a material are attracting attention. A metalorganic compound chemical vapor deposition method (hereafter referred as xe2x80x9cthe MOCVD methodxe2x80x9d) is well known as a method of growing crystalline GaN based semiconductor. The MOCVD method is one of the epitaxial growth methods with which a single crystalline compound semiconductor layer is grown on a substrate by supplying a gaseous metalorganic compound (hereafter referred as xe2x80x9cthe MO gasxe2x80x9d) with ammonia (NH3) gas as reaction gases (source gases) into a reaction chamber in which the substrate is located. In the case of growing, for example, GaN epitaxial layer, trimethylgallium (TMG), which is a MO gas as a group III gas, and NH3 gas as a group V gas are often used.
In order to manufacture semiconductor devices using the GaN based semiconductor crystals grown in this manner, noticeable improvement of crystallographic quality of the epitaxially grown layers is indispensable to improve the performance of the semiconductor devices.
The surface of, for example, a GaN layer grown directly on a sapphire (Al2O3) substrate by the MOCVD method has faults and the surface morphology is very poor due to the formation of hexagonal pyramid-like or hexagonal shaped patterns resulting in numerous irregularities on the surface. Manufacturing of semiconductor device such as a blue light emitting diode (hereafter referred as xe2x80x9cthe LEDxe2x80x9d) using the semiconductor crystalline layer having very poor surface morphology such that the numerous irregularities are present is practically impossible due to very low yield.
In order to solve these problems, there is proposed a method of growing a buffer layer of aluminum nitride (AlN) on the substrate before growing the GaN based semiconductor crystal fin (refer to Physics Letter vol. 48, 1986, p.353 and Japanese patent under provisional publication No.1990-229476) and a method of growing a buffer layer of GaN (refer to Japanese Journal of Applied Physics Vol. 30, 1991, pL1715 and Japanese patent under provisional publication No. 1992-297023). According to these method, a buffer layer of 10-120 nm in thickness is formed on the sapphire substrate at low temperature of 400-900xc2x0 C. These methods indicate a capability of improvement of the crystallographic quality and the surface morphology of the GaN semiconductor layer by growing GaN layer on the buffer layer.
However, the method described above is required to control very small quantity of feed gas, because the growing condition of buffer layer is limited strictly and further, because the film thickness must be controlled very accurately and strictly. The control of epitaxial growth does not accompany many difficulties when MO gas such as TMG gas is used as a Ga source. However, when gallium chloride (GaCl) formed by supplying hydrogen chloride (HCl) gas so as to contact with liquid gallium is used, the control range of feed quantity is much higher than that for the case of using MO gas, so that the control performance and the reproducibility are extremely low. In the case of obtaining a thick single crystalline GaN based semiconductor layer, the gas supply system of GaCl mentioned above is commonly used. One of the largest advantages is a cost of the raw material. There is a trial balance indicating that the cost of raw material can be lowered by two to three orders compared with MOCVD method. However, it is difficult to improve crystal quality and surface morphology of a laminated structure of the GaN based semiconductor formed on the thick single crystalline GaN based semiconductor layer grown by this method. Furthermore, the crystal quality and the surface morphology are not sufficiently good to fabricate a practical semiconductor laser diode, so that a noticeable improvement of the crystal quality is required. Especially, it is well known that the existence of large crystal defect called xe2x80x9cnanopipexe2x80x9d is a large problem in an improvement of semiconductor device particularly in reliability.
In the conventional light emitting device using a single crystalline GaN based semiconductor layer, because the sapphire substrate popularly employed for the epitaxial growth was insulating, it was not possible to dispose the electrode layer on the back surface. Hence, it is necessary to form anode and cathode electrodes on a same side of the surfaces, by exposing both n- and p-GaN based semiconductor layers, digging a trench for disposing one of the electrodes to the required contact layer. The electrode forms an ohmic contact electrode with a semiconductor layer appeared at the bottom of the trench. Not only a process of trench formation for disposing the electrode but also processes of wire bonding onto both of these two electrodes were required. In particular, since the GaN based semiconductor is a material difficult to etch, the formation of trench for forming the electrode is difficult and the fabrication processes are complicated. As the region of the trench for disposing the electrode occupied a certain area, the entire size could not be made small and so the yield was low. Such a difficulty was a serious problem especially in integrated structures.
The present invention is made in consideration of these circumstances and has an object to provide a method for preparing simply and in short time a substrate for epitaxial growth (hereinafter referred as xe2x80x9cthe epitaxial-substratexe2x80x9d) of a crystal having high crystallographic perfection and excellent surface morphology.
Another object of the present invention is to provide a method of manufacturing a semiconductor device wherein the electrical and optical behaviors are remarkably improved.
In order to achieve above-mentioned objects, first feature of the present invention lies in a method for preparing an epitaxial-substrate encompassing:
(a) growing epitaxially a first GaN based semiconductor layer on a bulk crystalline substrate (first process);
(b) growing epitaxially a GaN based semiconductor layer containing indium (hereinafter referred as xe2x80x9cthe InGaN based semiconductor layerxe2x80x9d) on the first GaN based semiconductor layer (second process);
(c) growing epitaxially a second GaN based semiconductor layer on the InGaN based semiconductor layer (third process); and
(d) separating the second GaN based semiconductor layer from the first GaN based semiconductor layer and provides the epitaxial-substrate (fourth process).
Here, xe2x80x9cthe bulk crystalline substratexe2x80x9d indicates a substrate such as the sapphire substrate, obtained from an ingot, or the bulk crystal, grown by a melt growth or a solution growth, by cutting down using a diamond blade and polishing into a geometry of xe2x80x9ca waferxe2x80x9d having a required thickness (for example, about a thickness of 100 to 300 xcexcm). The bulk crystalline substrate may be referred as xe2x80x9cthe bulk-substratexe2x80x9d hereinafter. According to the first feature of the present invention, an epitaxial-substrate having a high crystallographic perfection and an excellent surface morphology can be obtained simply and in a short time. For example, the defect density of the single crystalline GaN based semiconductor layer grown on the epitaxial-substrate is greatly reduced.
Second feature of the present invention inheres in a method of manufacturing a semiconductor device encompassing:
(a) growing epitaxially a first GaN based semiconductor layer on a bulk-substrate (first process);
(b) growing epitaxially an InGaN based semiconductor layer on the first GaN based semiconductor layer (second process);
(c) growing epitaxially a second GaN based semiconductor layer on the InGaN based semiconductor layer (third process);
(d) separating the second GaN based semiconductor layer from the bulk-substrate so as to obtain an epitaxial-substrate (fourth process);
(e) growing epitaxially a multilayered structure of single crystalline GaN based semiconductor layers on the epitaxial-substrate (fifth process);
(f) forming an electrode on the uppermost layer of multilayered structure (sixth process).
The first to fourth processes of the method of manufacturing the semiconductor device according to the second feature of the present invention are the same as the method for preparing the epitaxial-substrate according to the first feature described above, i.e. the semiconductor device is manufactured by adding the fifth and the sixth processes to the process sequence of the first feature. And the semiconductor device encompasses a stacked structure grown on the epitaxial-substrate obtained by the method according to the first feature. In the semiconductor device fabricated by the method of manufacturing the semiconductor device according to the second feature of the present invention, a significant improvement of the electrical and light emitting characteristics are obtained. Further, the operation lifetime of the light emitting device are improved. And, semiconductor devices with high yield can be easily manufactured as the manufacturing process is simplified.